Fun calculator
Birthday Paradox Calculator
Estimate the chance that at least two people in a group share a birthday.
Formula
Birthday match probability
P(match) = 1 - product((365 - i) / 365)
A group of 23 people has about a 50% chance of a shared birthday.
FAQs
Does this include leap years?+
No. It uses 365 equally likely birthdays for a simple estimate.
Why is the probability so high?+
Every pair of people creates a possible match, so comparisons grow quickly.
How does the Birthday Paradox Calculator calculate the result?+
It uses the Birthday match probability: P(match) = 1 - product((365 - i) / 365). A group of 23 people has about a 50% chance of a shared birthday.
What information do I need to use the Birthday Paradox Calculator?+
Estimate the chance that at least two people in a group share a birthday.
How accurate is the Birthday Paradox Calculator?+
Birthday Paradox Calculator applies the formula and assumptions shown on this page. Results may be rounded for readability, so verify changing rates, thresholds, medical guidance, or legal rules with the cited source or a qualified professional.
What should I check before using the Birthday Paradox Calculator result?+
Check that the units, dates, rates, and assumptions match your situation. Change one input at a time to understand which values have the largest effect on the result.
Fun guide
How to use the Birthday Paradox Calculator
Estimate the chance that at least two people in a group share a birthday. The page also explains the birthday match probability and shows a practical example: A group of 23 people has about a 50% chance of a shared birthday.
- 1
Enter your details
Enter the values for the birthday paradox calculator scenario you want to check.
- 2
Check the calculation
Review the result alongside the birthday match probability: P(match) = 1 - product((365 - i) / 365).
- 3
Compare scenarios
Change one or more inputs to see how they affect the birthday Paradox Calculator result before you use the estimate.
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